Even Gordon would lose his seat

Three-quarters of the Cabinet would be out too, including the four biggest guns: Justice Secretary Jack Straw, Home Secretary Jacqui Smith, Foreign Secretary David Miliband and Chancellor Alistair Darling.

Labour MPs plunged into panic mode this morning as they realised the scale of the earthquake.

While nobody regards by-elections as a guide to future general elections, the general reckoning was that no seat may be considered safe which has a majority of less than 4,000.

That leaves dozens of marginals - especially in the crucial South-East and London - looking doomed.

Ms Smith tops the list of big names facing defeat. Her majority of 2,716 in Redditch is cut to less than 2,000 by boundary changes and a swing of threeper cent would see her out.

Transport Secretary Ruth Kelly, MP for Bolton West, has a majority of 2,064, but after boundary changes that will shrink even further leaving her vulnerable-to a five per cent swing. Mr Darling-would vanish on an eight per cent swing along with Business Secretary John Hutton.

If the swing crept up to 14 per cent, Children's Secretary Ed Balls, one of Mr Brown's closest allies, would be out, too. Higher swings would see Brownites such as Douglas Alexander, and Des Browne bite the dust.

Only if the defeat were as bad as last night's - a far-fetched situation - would Mr Brown be taken out of the picture, as would the Blairite choice to succeed him, David Miliband. However, with dozens of Labour MPs nursing much smaller majorities around the South-East and London, far smaller and very realistic swings would take them out.

Higher Education minister Bill Rammell, former transport minister and actress Glenda Jackson, Government Whip Claire Ward and a clutch of ministerial aides are all very vulnerable.

Mr Rammell is MP for Harlow where his majority of 97 looks paper thin.

But Rudi Vis, the MP for Finchley and Golders Green, is the number one Labour target for the Conservatives. His 2005 majority of 741 is reduced to an estimated 31 after boundary changes.

Laura Moffatt, the PPS to Health Secretary Alan Johnson, is defending a majority of 37 in Crawley. Celia Barlow, the PPS to Innovations minister Ian Pearson, has only 420 votes in hand at Hove and Ms Jackson's 3,279 majority is reduced to a third of that by boundary changes at Hampstead & Kilburn.

Ms Ward, who was the youngest MP when she won Watford in 1997, is down to a 1,148 majority over the Liberal Democrats. Also vulnerable to the third party is Emily Thornberry in Islington South & Finsbury, whose 7,280 majority of 2005 is slashed to roughly 484 by boundary reforms.

Others in the "killing fields" of the South-East include former Guardian journalist Martin Linton, nursing a 163 majority in Battersea; Phyllis Starkey, with an estimated majority of 483 after boundary changes in Milton Keynes South; and Howard Stoate, holding Dartford by 706.